The mounting of an optical fiber in a connector member which has a centering surface at one end, in the form of a truncated external cone, is known for example, from applicant's U.S. Pat. No. 4,490,006, which is incorporated herein by reference. The invention can also be applied to other types of rotationally symmetrical fitting surfaces, e.g., right-cylindrical surfaces.
The known method makes it possible to use purely mechanical means to achieve centering of an optical fiber, so that it will be right on the axis of the surface of revolution. This centering will be with respect to the outer, essentially cylindridal surface of the fiber. For fibers for larger dimensions of step index type, this known method is completely satisfactory as regards accuracy. Difficulties arise, however, when it comes to centering optical fibers where the fiber core diameter is 50 .mu.m or less. Examples of such fibers are gradient index fibers in which the index of refraction gradually decreases from the center outwards, and single mode filters which have a core of approximately 10 .mu.m with one index of refraction and a jacket with a lower index of refraction.
The gradient index fiber is usually manufactured by coating the inside of a quartz tube from the vapor phase with a doped quartz sublimate which more or less continuously increases an index of refraction towards the center of the tube. Melting then causes the hole in the tube to close so that a solid quartz rod is obtained which is then drawn out to form a fiber. The single mode fiber is drawn from a double pot; molten quartz is allowed to run out of a hole forming a string which is surrounded by another type of molten quartz in an annular gap, and the running quartz string is then drawn out into a fiber. (These descriptions are of course only very rough descriptions.)
In both cases it is difficult to keep the fiber entirely rotationally symmetrical with regard to the center zone with the highest index of refraction, the core of the fiber. For example, a single mode fiber can have a diameter of 100 .mu.m and a core of 10 .mu.m in diameter which is often off-center by 3 .mu.m for a gradient index fiber, the usual diameter is 125 .mu.m, with an effective light transmission core diameter of 50 .mu.m. In this case, .+-.3% is a normal manufacturing tolerance. For connecting two gradient index fibers, the error must be less than 5 .mu.m to keep the loss under 1 dB. The problem of connecting single mode fibers is five times as difficult.
It will thus be appreciated that, when connecting optical fibers of this type, a centering which only takes into account the outer diameters cannot be considered satisfactory.
When connecting optical fibers with connectors, the Fresnel losses can amount to 0.35 dB, if the connection is made with an air gap. Better values can be obtained by filling the gap with a medium of matching index. This method must, however, often be rejected for practical reasons. It should be mentioned that a permanent juncture by fusing the fiber ends, which of course in principle should be without any loss, has in practice produced losses of several tenths of a decibel, due, among other things, to the asymmetrical position of the fiber core in relation to the outer diameter.